1. Field of the Invention
This invention relates to an image display apparatus, and more particularly, to an image display apparatus in which by utilizing a half-mirror, a hologram optical element or the like, a high picture quality virtual image of display information displayed on an image display device, serving as an image generator, and image information, such as a natural landscape or the like, are optically and spatially superimposed, so that the two kinds of information can be simultaneously observed on a large picture surface.
2. Description of the Related Art
Large-size CRT display apparatuses, projection TV apparatuses and the like have been used as image display apparatuses in which image information displayed on an image display device, such as a liquid-crystal display device or the like, is observed as a large-size image having a presence.
Such an image display apparatus requires a large space. Accordingly, if it is intended to install such an apparatus in a small room, a proper distance for observation cannot be secured. In addition, different people cannot see different programs.
In order to overcome the above-described problems, an image display apparatus has, for example, been proposed in Japanese Patent Laid-open Application (Kokai) No. 3-203478 (1991), in which, as shown in FIG. 1, a light beam from an image display device is directly guided to an eye (a pupil) of the observer using an optical system disposed in the vicintiy of the face of the observer so that large-size image information can be observed.
The principal portion of the apparatus shown in FIG. 1 will now be described. In FIG. 1, reference numerals 221R and 221L represent liquid-crystal color display devices for the right eye and the left eye of the observer, respectively. A part of the light beam from image information displayed on liquid-crystal color display device 221R (221L) is reflected by a trapezoid beam splitter (half-mirror) 222R (222L, not shown) disposed in front of the right (left) eye of the observer (not shown), and is incident upon a concave mirror lens 223 provided in the front side. The light beam reflected by the concave mirror lens 223 is incident upon the pupil of the observer after passing through the beam splitter 222R (222L).
Thus, the observer can observe the image information displayed on the liquid-crystal color display devices 221R and 221L as a virtual image at a predetermined position in front of the concave mirror lens 223.
Various kinds of image display apparatuses of this kind have been proposed in which display information providing a parallax between the right and left eyes of the observer is displayed so that the observer can observe a stereoscopic image.
In the conventional image display apparatus shown in FIG. 1, display information from each display device is guided to the corresponding pupil of the observer via the half-mirror and a semitransparent reflecting concave mirror lens. The display information and image information, such as an external landscape or the like, are superimposed using a beam combiner, so that the observer can simultaneously observe the two kinds of information.
The light beam having the display information from the display device is reflected by the half-mirror and the reflecting concave mirror lens. Hence, if the reflectivity (transmittance) of these components is assumed to be 50%, the overall light-beam utilization efficiency is (0.5).sup.3 =0.125. That is, the utilization efficiency of the light beam is poor, and therefore the display information becomes obscure.
In the conventional image display apparatus, if it is intended to provide large display information, the curvature of the semitransparent reflecting mirror lens must be reduced so as to increase the optical reflective power. As a result, the curved surface of the mirror lens deflects the transmitted light beam, thereby causing a decrease in the superimposing property.
Furthermore, since the display devices comprising the liquid-crystal displays must be provided in the vicinity of the reflecting concave mirror lens, the reflected light beam is eclipsed, and therefore it becomes difficult to observe the entire picture surface in an excellent condition.
In an apparatus in which display information having a parallax is displayed from display devices by providing two kinds of image information for the right and left eyes of the observer so that the observer can observe a stereoscopic image, the depth of field of the eyes of the observer must coincide with the range of image depth of the sterepscopic display information.
In this case, in order to provide an image display apparatus having a sufficient stereoscopic effect, it is necessary to display image information as bright as possible, to increase the depth of field of the eyes of the observer by reducing the size of the iris of each eye of the observer, and to increase the range of image depth of the stereoscopically displayed image.
When a portable stereoscopic-image display apparatus is used, the light-beam utilization efficiency must be further increased in order to display bright image information with a small electric power consumption.
However, it is very difficult to obtain bright image information by increasing the light-beam utilization efficiency. Furthermore, when a large picture surface is displayed, distortion in the displayed image becomes prominent, causing the observer to feel that the image is unnatural.